1. Field of the Invention
The present invention relates to mobile devices. More particularly, the present invention relates to multi-mode multi-radio mobile devices.
2. Description of the Related Art
In recent years, cellular communications systems have been widely deployed throughout many parts of the world. The term “cellular” refers to communications systems, such as the Advance Mobile Phone Service (“AMPS”), that divide a geographic region into sections known as cells. The purpose of this division is to make the most use out of a limited number of transmission frequencies. In many implementations each connection, or conversation, requires its own dedicated frequency and the total number of available frequencies is on the order of 1,000. To support more than 1,000 simultaneous conversations, cellular systems allocate a set number of frequencies for each cell. Two cells can use the same frequency for different conversations so long as the cells are not adjacent to each other.
Several competing cellular systems exist, including GSM and CDMA. GSM is an acronym for Global System for Mobile Communications, one of the leading digital cellular systems. GSM uses narrowband Time Division Multiple Access (“TDMA”), which divides a radio frequency into time slots and then allocates slots to multiple calls. In this way, a single frequency can support multiple, simultaneous data channels. GSM was first introduced in 1991. As of the end of 1997, GSM service was available in more than 100 countries and has become the de facto standard in Europe and Asia.
CDMA is an acronym for Code-Division Multiple Access, which does not assign a specific frequency to each user. Instead, every channel uses the full available spectrum. Individual conversations are encoded with a pseudo-random digital sequence.
The General Packet Radio Service (“GPRS”) is a standard for wireless communications that runs at speeds up to 115 kilobits per second, compared with current GSM systems' 9.6 kilobits per second. GPRS supports a wide range of bandwidths and is an efficient use of limited bandwidth. Therefore, GPRS is particularly suited for sending and receiving small bursts of data, such as e-mail and Web browsing, as well as large volumes of data.
Some wireless networks are 802.11 networks. “802.11” refers to a family of specifications developed by the IEEE for wireless local area network (“WLAN”) technology. 802.11 specifies an over-the-air interface between a wireless client and a base station or between two wireless clients. There are several specifications in the 802.11 family:
802.11—applies to wireless LANs and provides 1 or 2 Mbps transmission in the 2.4 GHz band using either frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS).
802.11a—an extension to 802.11 that applies to wireless LANs and provides up to 54 Mbps in the 5 GHz band. 802.11a uses an orthogonal frequency division multiplexing encoding scheme rather than FHSS or DSSS.
802.11b (also referred to as 802.11 High Rate or Wi-Fi)—an extension to 802.11 that applies to wireless LANS and provides 11 Mbps transmission (with a fallback to 5.5, 2 and 1 Mbps) in the 2.4 GHz band. 802.11b uses only DSSS. 802.11b was a 1999 ratification to the original 802.11 standard, allowing wireless functionality comparable to Ethernet.
802.11g—applies to wireless LANs and provides more than 20 Mbps in the 2.4 GHz band.
Many cellular telephones, personal digital assistants (“PDAs”) and similar devices are now configured with more than one radio in order to operate in more than one mode. This configuration allows the devices to use more than one wireless access network. Such devices will be referred to herein as multi-mode multi-radio mobile devices (“MMMDs”). MMMDs are capable of utilizing multiple wireless access networks for connectivity.
Wireless devices, including MMMDs, generally become aware of wireless network connectivity by periodically scanning for access points (“APs”) or base stations, which typically transmit beacons periodically. The wireless device may then use wireless link quality metrics, or other metrics, to assess the different connectivity options.
For example, in 802.11 WLAN networks, a WLAN client may perform active or passive scanning. In active scanning mode, the wireless device turns on its radio, tunes to a particular channel and sends a probe request. The wireless device then waits for a probe response and repeats this process for all available channels until an AP is found. In passive scanning mode the wireless device turns on the radio, tunes to a channel and waits to hear the beacon from an AP. The wireless device then repeats this process for all available channels until an AP is found.
Having multiple radios active and performing periodic scanning is expensive in terms of power consumption, especially if performed frequently. It may also increase the time taken to detect and access a wireless network. It would be desirable for MMMDs to have more efficient methods and devices for evaluating and/or establishing wireless network connectivity.